Heretofore known means utilized for cooling solids to very low temperatures (-200 C. or lower) all have well understood drawbacks. Cryogenic fluids are messy, expensive, and difficult to contain, regulate and incorporate into automated processes. Sterling cycle pumps are also expensive, are not sufficiently reliable in many application, and cause vibration.
The need for reliable and low cost means for cooling solids to such low temperatures is, however, growing. For instance, infra-red viewers require cooled sensors in order to be able to image purely ambient thermal radiation. Currently, these devices employ closed-cycle refrigerators that is both expensive and bulky, thus making use of such devices impractical in a variety of every-day applications for which they might otherwise be used. Superconductor integrated circuits such as those used in some computers and elsewhere could be more widely employed if the need for expensive cryogenic refrigeration systems used with such circuits could be eliminated.
In the last decade there has been substantial progress in the field of laser cooling of atomic gases. Temperatures as low as a millionth of a degree Kelvin have been obtained, for instance, in a Cesium vapor. A number of different mechanisms have been successfully employed in cooling such vapors. In each the basic idea is the same, an intense beam of monochromatic laser light being directed into the vapor to thereby take advantage of a mechanism internal to the atom to cause it to absorb a photon from the laser beam and then reemit a photon of slightly higher frequency (i.e., higher energy). The net loss of energy from the atom causes the individual atom, and eventually the entire gas of atoms, to slow down, that is, to become colder. These methods for cooling atomic vapors have, however, had very limited industrial applicability, the methods only being effective for extremely low density atomic vapors.
Various theoretical approaches to optically cooling solids have been heretofore suggested. For example, optical refrigeration of diodes by flowing a current into a light-emitting diode and generating photons with greater than eV worth of energy each has been heretofore suggested, but as a practical matter is unlikely to be effective in view of the joule heating associated with electrical current flow.
Other paradigms of optical cooling of media have been theoretically investigated, including sending a low energy photon into a medium and getting a high energy photon out with resultant cooling. However, none have suggested any particular mechanism or device for causing emitted photons from a solid to be of higher energy than photons entering the solid, nor have they confronted the particular technical problems that have seemed to make such theoretical optical approaches to cooling of a solid impractical.